Pediatric Lymphoid and Histiocytic Lesions in the Head and Neck.

Lymphoid and histiocytic lesions of the head and neck in pediatric patients is a fascinating topic as most of these lesions are benign, but that the neoplastic cases are essential to diagnose accurately for appropriate treatment. It is thought that 90% of children will have palpable lymph nodes between the ages of 4 to 8; most, but not all, are non-malignant and some resolve spontaneously without treatment. This paper will look at many of the benign and malignant lesions of both lymphocytic and histiocytic origin that present in the head and neck of children focusing on their diagnostic criteria. There is a very pertinent discussion of nonmalignant lymphoid proliferations, as infections and other reactive conditions dominate the pathology of pediatric lymphohistiocytic head and neck lesions. Discussion of those lymphomas which arise more frequently in the head and neck focuses on those seen in children and young adults such as classic Hodgkin lymphoma and Burkitt lymphoma, as well as new more controversial entities such as pediatric-type follicular lymphoma. Histiocytic lesions, both benign and malignant, are described and may be challenging to diagnose.

Effects of neratinib on health-related quality of life in women with HER2-positive early-stage breast cancer: longitudinal analyses from the randomized phase III ExteNET trial.

BACKGROUND: We report longitudinal health-related quality-of-life (HRQoL) data from the international, randomized, double-blind, placebo-controlled phase III ExteNET study, which demonstrated an invasive disease-free survival benefit of extended adjuvant therapy with neratinib over placebo in human epidermal growth factor receptor-2-positive early-stage breast cancer. PATIENTS AND METHODS: Women (N = 2840) with early-stage HER2-positive breast cancer who had completed trastuzumab-based adjuvant therapy were randomly assigned to neratinib 240 mg/day or placebo for 12 months. HRQoL was an exploratory end point. Patients completed the Functional Assessment of Cancer Therapy-Breast (FACT-B) and EuroQol 5-Dimensions (EQ-5D) questionnaires at baseline and months 1, 3, 6, 9, and 12. Changes from baseline were compared using analysis of covariance with no imputation for missing values. Sensitivity analyses used alternative methods. Changes in HRQoL scores were regarded as clinically meaningful if they exceeded previously reported important differences (IDs). RESULTS: Of the 2840 patients (intention-to-treat population), 2407 patients were evaluable for FACT-B (neratinib, N = 1171; placebo, N = 1236) and 2427 patients for EQ-5D (neratinib, N = 1186; placebo, N = 1241). Questionnaire completion rates exceeded 85%. Neratinib was associated with a decrease in global HRQoL scores at month 1 compared with placebo (adjusted mean differences: FACT-B total, -2.9 points; EQ-5D index, -0.02), after which between-group differences diminished at later time-points. Except for the FACT-B physical well-being (PWB) subscale at month 1; all between-group differences were less than reported IDs. The FACT-B breast cancer-specific subscale showed small improvements with neratinib at months 3-9, but all were less than IDs. Sensitivity analyses exploring missing data did not change the results. CONCLUSIONS: Extended adjuvant neratinib was associated with a transient, reversible decrease in HRQoL during the first month of treatment, possibly linked to treatment-related diarrhea. With the exception of the PWB subscale at month 1, all neratinib-related HRQoL changes did not reach clinically meaningful thresholds. ClinicalTrials.gov: NCT00878709.

Teacher Knowledge for Active-Learning Instruction: Expert-Novice Comparison Reveals Differences.

Active-learning strategies can improve science, technology, engineering, and mathematics (STEM) undergraduates' abilities to learn fundamental concepts and skills. However, the results instructors achieve vary substantially. One explanation for this is that instructors commonly implement active learning differently than intended. An important factor affecting how instructors implement active learning is knowledge of teaching and learning. We aimed to discover knowledge that is important to effective active learning in large undergraduate courses. We developed a lesson-analysis instrument to elicit teacher knowledge, drawing on the theoretical construct of teacher noticing. We compared the knowledge used by expert (n = 14) and novice (n = 29) active-learning instructors as they analyzed lessons. Experts and novices differed in what they noticed, with experts more commonly considering how instructors hold students accountable, topic-specific student difficulties, whether the instructor elicited and responded to student thinking, and opportunities students had to generate their own ideas and work. Experts were also better able to support their lesson analyses with reasoning. This work provides foundational knowledge for the future design of preparation and support for instructors adopting active learning. Improving teacher knowledge will improve the implementation of active learning, which will be necessary to widely realize the potential benefits of active learning in undergraduate STEM.

Quantum criticality at the superconductor-insulator transition revealed by specific heat measurements.

The superconductor-insulator transition (SIT) is considered an excellent example of a quantum phase transition that is driven by quantum fluctuations at zero temperature. The quantum critical point is characterized by a diverging correlation length and a vanishing energy scale. Low-energy fluctuations near quantum criticality may be experimentally detected by specific heat, cp, measurements. Here we use a unique highly sensitive experiment to measure cp of two-dimensional granular Pb films through the SIT. The specific heat shows the usual jump at the mean field superconducting transition temperature marking the onset of Cooper pairs formation. As the film thickness is tuned towards the SIT, is relatively unchanged, while the magnitude of the jump and low-temperature specific heat increase significantly. This behaviour is taken as the thermodynamic fingerprint of quantum criticality in the vicinity of a quantum phase transition.

Use and outcomes of venous thromboembolism prophylaxis after spinal fusion surgery.

BACKGROUND: The number of spinal fusion operations in the USA is rapidly rising, but little is known about optimal venous thromboembolism prophylaxis after spinal surgery. OBJECTIVES: To examine the use of and outcomes associated with venous thromboembolism prophylaxis after spinal fusion surgery in a cohort of 244 US hospitals. PATIENTS/METHODS: We identified all patients with a principal procedure code for spinal fusion surgery in hospitals participating in the Premier Perspective database from 2003 to 2005, and searched for receipt of pharmacologic prophylaxis (subcutaneous unfractionated heparin, low molecular weight heparin, or fondaparinux) and/or mechanical prophylaxis (compression devices and elastic stockings) within the first 7 days after surgery. We also searched for discharge diagnosis codes for venous thromboembolism and postoperative hemorrhage during the index hospitalization and within 30 days after surgery. RESULTS: Among 80,183 spinal fusions performed during the time period, cervical fusions were the most common (49.0%), followed by lumbar fusions (47.8%). Thromboembolism prophylaxis was administered to 60.6% of patients within the first week postoperatively, with the most frequent form being mechanical prophylaxis alone (47.6%). Of the 244 hospitals, 26.2% provided prophylaxis to ≥ 90% of their patients undergoing spinal fusion; however, 33.2% provided prophylaxis to fewer than 50% of their patients. The rate of diagnosed venous thromboembolism within 30 days after surgery was 0.45%, and the rate of postoperative hemorrhage was 1.1%. CONCLUSIONS: Substantial variation exists in the use of thromboembolism prophylaxis after spinal fusion surgery in the USA. Nevertheless, overall rates of diagnosed thromboembolism after spinal fusion appear to be low.

Republished paper: Improving safety culture on adult medical units through multidisciplinary teamwork and communication interventions: the TOPS Project.

AIM: The goal of this project was to improve unit-based safety culture through implementation of a multidisciplinary (pharmacy, nursing, medicine) teamwork and communication intervention. METHOD: The Agency for Healthcare Research and Quality Hospital Survey on Patient Safety Culture was used to determine the impact of the training with a before-after design. RESULTS: Surveys were returned from 454 healthcare staff before the training and 368 staff 1 year later. Five of eleven safety culture subscales showed significant improvement. Nurses perceived a stronger safety culture than physicians or pharmacists. CONCLUSION: While it is difficult to isolate the effects of the team training intervention from other events occurring during the year between training and postevaluation, overall the intervention seems to have improved the safety culture on these medical units.

Improving safety culture on adult medical units through multidisciplinary teamwork and communication interventions: the TOPS Project.

AIM: The goal of this project was to improve unit-based safety culture through implementation of a multidisciplinary (pharmacy, nursing, medicine) teamwork and communication intervention. METHOD: The Agency for Healthcare Research and Quality Hospital Survey on Patient Safety Culture was used to determine the impact of the training with a before-after design. RESULTS: Surveys were returned from 454 healthcare staff before the training and 368 staff 1 year later. Five of eleven safety culture subscales showed significant improvement. Nurses perceived a stronger safety culture than physicians or pharmacists. CONCLUSION: While it is difficult to isolate the effects of the team training intervention from other events occurring during the year between training and postevaluation, overall the intervention seems to have improved the safety culture on these medical units.

The serotonin1A receptor gene as a genetic and prenatal maternal environmental factor in anxiety.

Low serotonin(1A) receptor (5-HT(1A)R) binding is a risk factor for anxiety and depression, and deletion of the 5-HT(1A)R results in anxiety-like behavior in mice. Here we show that anxiety-like behavior in mice also can be caused, independently of the offspring's own 5-HT(1A)R genotype, by a receptor deficit in the mother: a nongenetic transmission of a genetic defect. Some of the nongenetically transmitted anxiety manifestations were acquired prenatally and linked to a delay in dentate gyrus maturation in the ventral hippocampus of the offspring. Both the developmental delay and the anxiety-like phenotype were phenocopied by the genetic inactivation of p16(ink4a) encoding a cyclin-dependent kinase inhibitor implicated in neuronal precursor differentiation. No maternal 5-HT(1A)R genotype-dependent anxiety developed when the strain background was switched from Swiss Webster to C57BL/6, consistent with the increased resilience of this strain to early adverse environment. Instead, all anxiety manifestations were caused by the offspring's own receptor deficiency, indicating that the genetic and nongenetic effects converge to common anxiety manifestations. We propose that 5-HT(1A)R deficit represents a dual risk for anxiety and that vulnerability to anxiety associated with genetic 5-HT(1A)R deficiency can be transmitted by both genetic and nongenetic mechanisms in a population. Thus, the overall effect of risk alleles can be higher than estimated by traditional genetic assays and may contribute to the relatively high heritability of anxiety and psychiatric disorders in general.

Recommendations for locus-specific databases and their curation.

Expert curation and complete collection of mutations in genes that affect human health is essential for proper genetic healthcare and research. Expert curation is given by the curators of gene-specific mutation databases or locus-specific databases (LSDBs). While there are over 700 such databases, they vary in their content, completeness, time available for curation, and the expertise of the curator. Curation and LSDBs have been discussed, written about, and protocols have been provided for over 10 years, but there have been no formal recommendations for the ideal form of these entities. This work initiates a discussion on this topic to assist future efforts in human genetics. Further discussion is welcome.

A structured simple form for ordering genetic tests is needed to ensure coupling of clinical detail (phenotype) with DNA variants (genotype) to ensure utility in publication and databases.

Researchers and clinicians ideally need instant access to all the variation in their gene/locus of interest to efficiently conduct their research and genetic healthcare to the highest standards. Currently much key data resides in the laboratory books or patient records around the world, as there are many impediments to submitting this data. It would be ideal therefore if a semiautomated pathway was available, with a minimum of effort, to make the deidentified data publicly available for others to use. The Human Variome Project (HVP) meeting listed 96 recommendations to work toward this situation. This article is planned to initiate a strategy to enhance the collection of phenotype and genotype data from the clinician/diagnostic laboratory nexus. Thus, the aim is to develop universally applicable forms that people can use when investigating patients for each inherited disease, to assist in satisfying many of the recommendations of the HVP Meeting [Cotton et al., 2007]. We call for comment and collaboration in this article.

Rare primary CNS anaplastic large cell lymphoma in an immunocompetent adult: a clinical-pathologic case report and review case of the literature.

OBJECTIVE AND IMPORTANCE: Isolated anaplastic large cell lymphoma (ALCL) presenting in the primary central nervous system is distinctly uncommon. The authors describe a case that clinically and radiographically simulated a primary glial neoplasm. CLINICAL PRESENTATION: A 39-year-old immunocompetent male presented with seizures and a rapidly enlarging right occipital/parietal lesion. Magnetic resonance images demonstrated a right occipitoparietal lesion, hypodense on T1WI, with patchy contrast enhancement with gadolinium and significant white matter edema pattern on T2WI along with mass effect and midline shift. INTERVENTION: The patient underwent a frameless stereotactic assisted needle biopsy. There appeared to be a clear demarcation between white matter and tumor with no obvious necrosis. Biopsy showed a proliferation of single cells and poorly cohesive groups of cells with large, pleomorphic nuclei, many containing prominent nucleoli, and a moderate amount of cytoplasm. Immunohistochemical staining revealed CD-30 and ALK-positivity typical of ALCL, a rare form of T-cell lymphoma. An extensive workup revealed neither systemic disease nor evidence of immunocompromise. CONCLUSION: Reported in less than 20 patients, primary ALCL in an immunocompetent patient is rarely found intracranially; however, its ability to mimic glial neoplasms as well as other pathologies underlines its importance.

A note on competing risks in survival data analysis.

Survival analysis encompasses investigation of time to event data. In most clinical studies, estimating the cumulative incidence function (or the probability of experiencing an event by a given time) is of primary interest. When the data consist of patients who experience an event and censored individuals, a nonparametric estimate of the cumulative incidence can be obtained using the Kaplan-Meier method. Under this approach, the censoring mechanism is assumed to be noninformative. In other words, the survival time of an individual (or the time at which a subject experiences an event) is assumed to be independent of a mechanism that would cause the patient to be censored. Often times, a patient may experience an event other than the one of interest which alters the probability of experiencing the event of interest. Such events are known as competing risk events. In this setting, it would often be of interest to calculate the cumulative incidence of a specific event of interest. Any subject who does not experience the event of interest can be treated as censored. However, a patient experiencing a competing risk event is censored in an informative manner. Hence, the Kaplan-Meier estimation procedure may not be directly applicable. The cumulative incidence function for an event of interest must be calculated by appropriately accounting for the presence of competing risk events. In this paper, we illustrate nonparametric estimation of the cumulative incidence function for an event of interest in the presence of competing risk events using two published data sets. We compare the resulting estimates with those obtained using the Kaplan-Meier approach to demonstrate the importance of appropriately estimating the cumulative incidence of an event of interest in the presence of competing risk events.

The dissociation of acetylcholine from open nicotinic receptor channels.

Ligand-gated ion channels bind agonists with higher affinity in the open than in the closed state. The kinetic basis of this increased affinity has remained unknown, because even though the rate constants of agonist association to and dissociation from closed receptors can be estimated with reasonable certainty, the kinetics of the binding steps in open receptors have proven to be elusive. To be able to measure the agonist-dissociation rate constant from open muscle nicotinic receptors, we increased the probability of ligand unbinding from the open state by engineering a number of mutations that speed up opening and slow down closing but leave the ligand-binding properties unchanged. Single-channel patch-clamp recordings from the wild-type and mutant constructs were performed at very low concentrations of acetylcholine (ACh). The durations of individual channel activations were analyzed assuming that "bursts" of fully liganded (diliganded) receptor openings can be terminated by ligand dissociation from the closed or open state (followed by fast closure) or by desensitization. This analysis revealed that ACh dissociates from diliganded open receptors at approximately 24 s(-1), that is, approximately 2,500 times more slowly than from diliganded closed receptors. This change alone without a concomitant change in the association rate constant to the open state quantitatively accounts for the increased equilibrium affinity of the open channel for ACh. Also, the results predict that both desensitization and ACh dissociation from the open state frequently terminate bursts of openings in naturally occurring gain-of-function mutants (which cause slow-channel congenital myasthenia) and therefore would contribute significantly to the time course of the endplate current decay in these disease conditions.

Endoscopic sinus surgery in cystic fibrosis: do patients benefit from surgery?

OBJECTIVE: To examine the effects of endoscopic sinus surgery on the pulmonary status of cystic fibrosis (CF) patients through the objective parameters of steroid use, pulmonary function tests (PFTs), and inpatient hospital days (IHDs). METHODS: Retrospective chart review of all patients with CF who underwent endoscopic sinus surgery from 1993 to 1999 at a tertiary care children's hospital. Preoperative pulmonary function, inhaler and steroid use, and IHDs were compared to postoperative parameters within a 1-year period. RESULTS: Sixty-six patients, including eight lung transplant patients, underwent a total of 112 endoscopic sinus surgery procedures; 25 patients underwent more than one procedure. Patients were taking oral steroids preoperatively in 28% of procedures and inhaled steroids in 40%. Postoperatively, there was no statistically significant change in oral or inhaled steroid use, or in postoperative pulmonary function. If the index hospitalization, which was often for reasons not related to sinus disease, was considered part of the preoperative time period, endoscopic sinus surgery (ESS) was noted to result in a marked reduction (9.5 days (adjusted), P=0.001) in hospital days during the subsequent 6 months. If the date of the procedure alone was used to define pre- and postoperative time periods, the reduction in postoperative days was more modest and not statistically significant (3.5 days (adjusted), P=0.21). CONCLUSIONS: Although we found no statistically significant difference in PFTs, or steroid requirements following ESS, ESS may have resulted in a reduced need for hospitalization in the 6 months following the procedure. Future prospective studies in a larger number of patients and using more detailed outcome measures are needed to better evaluate the effects of endoscopic sinus surgery in pediatric patients with CF.

Evaluation of growth and hormonal status in patients referred to the International Fanconi Anemia Registry.

OBJECTIVES: 1) To determine the extent of short stature in patients with Fanconi anemia (FA); 2) to determine the extent and nature of endocrinopathy in FA; 3) to assess the impact on height of any endocrinopathies in these patients; and 4) to study the correlation, if any, between height, endocrinopathy, and FA complementation group. STUDY DESIGN: Fifty-four patients with FA, 30 males and 24 females from 47 unrelated families, were prospectively evaluated in a Pediatric Clinical Research Center. The patients ranged in age from 0.1-31.9 years, with the mean age at assessment 8.6 years. RESULTS: Endocrine abnormalities were found in 44 of the 54 FA patients tested (81%), including short stature, growth hormone (GH) insufficiency, hypothyroidism, glucose intolerance, hyperinsulinism, and/or overt diabetes mellitus. Twenty-one of 48 (44%) participants had a subnormal response to GH stimulation; 19 of 53 (36%) had overt or compensated hypothyroidism, while 8 of 40 participants had reduced thyroid-hormone binding. Two patients were diabetic at the time of study; impaired glucose tolerance was found in 8 of 40 patients (25%), but most surprisingly, hyperinsulinemia was present in 28 of 39 (72%) participants tested. Significantly, spontaneous overnight GH secretion was abnormal in all patients tested (n = 13). In addition, participants demonstrated a tendency toward primary hypothyroidism with serum tetraiodothyronine levels at the lower range of normal, while also having thyrotropin (thyroid-stimulating hormone) levels at the high end of normal. Sixteen patients were assigned to FA complementation group A, (FA-A), 12 to FA-C, and 5 to FA-G; 10 of the 12 participants in FA-C were homozygous for a mutation in the intron-4 donor splice site of the FANCC gene. Patients in groups FA-A and FA-G were relatively taller than the group as a whole (but still below the mean for the general population), whereas those in FA-C had a significantly reduced height for age. GH response to stimulation testing was most consistently normal in participants from FA-G, but this did not reach statistical significance. The tendency toward hypothyroidism was more pronounced in participants belonging to complementation groups FA-C and FA-G, whereas insulin resistance was most evident in patients in FA-G, and least evident in those in FA-C. Short stature was a very common finding among the patients with a mean height >2 standard deviations below the reference mean (standard deviation score: -2.35 +/- 0.28). Patients with subnormal GH response and those with overt or compensated hypothyroidism were shorter than the group with no endocrinopathies. The heights of those participants with glucose or insulin abnormalities were less severely affected than those of normoglycemic, normoinsulinemic participants, although all were significantly below the normal mean. The mean height standard deviation score of patients with entirely normal endocrine function was also >2 standard deviations below the normal mean, demonstrating that short stature is an inherent feature of FA. CONCLUSION: Endocrinopathies are a common feature of FA, primarily manifesting as glucose/insulin abnormalities, GH insufficiency, and hypothyroidism. Although short stature is a well-recognized feature of FA, 23 patients (43%) were within 2 standard deviations, and 5 of these (9% of the total) were actually above the mean for height for the general population. Those patients with endocrine dysfunction are more likely to have short stature. These data indicate that short stature is an integral feature of FA, but that superimposed endocrinopathies further impact on growth. The demonstration of abnormal endogenous GH secretion may demonstrate an underlying hypothalamic-pituitary dysfunction that results in poor growth.

Physician views on caring for hospitalized patients and the hospitalist model of inpatient care.

We surveyed 241 board-certified internists affiliated with a large teaching hospital (Boston, Mass) before implementing a hospitalist service to determine attitudes towards providing inpatient care and the hospitalist model. Of physicians surveyed, 66% responded. Most disagreed that inpatient care is "an inefficient use of my time," only 10% felt a hospitalist service would improve patient satisfaction, and 54% felt it would hurt patient-doctor relationships. Multivariable analyses suggest that physicians physically furthest from their inpatient site were had more favorable attitudes toward the hospitalist model; more experienced and busier physicians were more negative. Future investigations should determine strategies for implementing the hospitalist model which address physicians' concerns.

Somatic mosaicism in Fanconi anemia: evidence of genotypic reversion in lymphohematopoietic stem cells.

Somatic mosaicism has been observed previously in the lymphocyte population of patients with Fanconi anemia (FA). To identify the cellular origin of the genotypic reversion, we examined each lymphohematopoietic and stromal cell lineage in an FA patient with a 2815-2816ins19 mutation in FANCA and known lymphocyte somatic mosaicism. DNA extracted from individually plucked peripheral blood T cell colonies and marrow colony-forming unit granulocyte-macrophage and burst-forming unit erythroid cells revealed absence of the maternal FANCA exon 29 mutation in 74.0%, 80.3%, and 86.2% of colonies, respectively. These data, together with the absence of the FANCA exon 29 mutation in Epstein-Barr virus-transformed B cells and its presence in fibroblasts, indicate that genotypic reversion, most likely because of back mutation, originated in a lymphohematopoietic stem cell and not solely in a lymphocyte population. Contrary to a predicted increase in marrow cellularity resulting from reversion in a hematopoietic stem cell, pancytopenia was progressive. Additional evaluations revealed a partial deletion of 11q in 3 of 20 bone marrow metaphase cells. By using interphase fluorescence in situ hybridization with an MLL gene probe mapped to band 11q23 to identify colony-forming unit granulocyte-macrophage and burst-forming unit erythroid cells with the 11q deletion, the abnormal clone was exclusive to colonies with the FANCA exon 29 mutation. Thus, we demonstrate the spontaneous genotypic reversion in a lymphohematopoietic stem cell. The subsequent development of a clonal cytogenetic abnormality in nonrevertant cells suggests that ex vivo correction of hematopoietic stem cells by gene transfer may not be sufficient for providing life-long stable hematopoiesis in patients with FA.

K(+) occupancy of the N-methyl-d-aspartate receptor channel probed by Mg(2+) block.

The single-channel kinetics of extracellular Mg(2+) block was used to probe K(+) binding sites in the permeation pathway of rat recombinant NR1/NR2B NMDA receptor channels. K(+) binds to three sites: two that are external and one that is internal to the site of Mg(2+) block. The internal site is approximately 0.84 through the electric field from the extracellular surface. The equilibrium dissociation constant for this site for K(+) is 304 mM at 0 mV and with Mg(2+) in the pore. The occupancy of any one of the three sites by K(+) effectively prevents the association of extracellular Mg(2+). Occupancy of the internal site also prevents Mg(2+) permeation and increases (by approximately sevenfold) the rate constant for Mg(2+) dissociation back to the extracellular solution. Under physiological intracellular ionic conditions and at -60 mV, there is approximately 1,400-fold apparent decrease in the affinity of the channel for extracellular Mg(2+) and approximately 2-fold enhancement of the apparent voltage dependence of Mg(2+) block caused by the voltage dependence of K(+) occupancy of the external and internal sites.

Na(+) occupancy and Mg(2+) block of the n-methyl-d-aspartate receptor channel.

The effect of extracellular and intracellular Na(+) on the single-channel kinetics of Mg(2+) block was studied in recombinant NR1-NR2B NMDA receptor channels. Na(+) prevents Mg(2+) access to its blocking site by occupying two sites in the external portion of the permeation pathway. The occupancy of these sites by intracellular, but not extracellular, Na(+) is voltage-dependent. In the absence of competing ions, Mg(2+) binds rapidly (>10(8) M(-1)s(-1), with no membrane potential) to a site that is located 0.60 through the electric field from the extracellular surface. Occupancy of one of the external sites by Na(+) may be sufficient to prevent Mg(2+) dissociation from the channel back to the extracellular compartment. With no membrane potential; and in the absence of competing ions, the Mg(2+) dissociation rate constant is >10 times greater than the Mg(2+) permeation rate constant, and the Mg(2+) equilibrium dissociation constant is approximately 12 microM. Physiological concentrations of extracellular Na(+) reduce the Mg(2+) association rate constant approximately 40-fold but, because of the "lock-in" effect, reduce the Mg(2+) equilibrium dissociation constant only approximately 18-fold.